Piezoelectric devices are known which can be used as a power source and are particularly useful for remote applications where other power sources such as batteries or generators are impractical. In common applications, the energy of the piezoelectric device is harvested and stored in an energy storage element. Piezoelectric devices internally produce energy as the device vibrates. The energy available from a piezoelectric device increases as the square of the voltage. The relationship is that the energy (E) stored in a device equals ½ CV2, C being the capacitance of the device and V being the voltage. Although the energy of the device increases as a squared function, the charge available goes up linearly.
Common circuits to discharge piezoelectric devices use capacitors as the power storage element. The disadvantage of discharging a piezoelectric device directly to a capacitor is that the energy generated by the piezoelectric device is not fully utilized. To improve the transfer and storage of a piezoelectric device's energy, inductors (e.g., coils) are disposed in the flow path to the energy storage element (e.g., the capacitor). The inductors store a portion of the energy discharged from the piezoelectric device and this energy can be harvested once the piezoelectric charge dissipates.
Piezoelectric devices generally produce energy at high frequency and low amperage. The intent of the energy storage elements, therefore, is to store this energy in a device capable of providing the higher amperage necessary to operate other devices (e.g., sensor systems and remote devices) which are an increasing area of use for piezoelectric devices. Common piezoelectric device energy harvesting circuits include diodes and inductors arranged in parallel circuit paths across the terminals of the piezoelectric device. Switches are commonly disposed in each parallel circuit path which are timed to match the driving frequency inducing piezoelectric device energy generation. The disadvantage of using switches is that each switch and its controlling circuitry dissipates a portion of the already low charge of the piezoelectric device before it reaches the energy storage element.
It is therefore desirable to provide a circuit for discharging piezoelectric devices which eliminates the switches such that the energy normally lost to operate the switches is retained and saved by the energy storage element. It is also desirable to provide a passive piezoelectric energy harvesting circuit to reduce circuit cost and decrease circuit complexity.